MongoDB学习资料六---MongoDB数据库的索引操作（转）

Mongodb数据库的索引操作很简单，只需要把作为条件的字段设置为索引即可

CODE:

> use user
switched to db user
> show collections
system.indexes
u_info
u_setting
> db.system.indexes.find();   这是默认的索引（默认为_id为索引）
{ "name" : "_id_", "ns" : "user.u_info", "key" : { "_id" : ObjectId("000000000000000000000000") } }
{ "name" : "_id_", "ns" : "user.u_setting", "key" : { "_id" : ObjectId("000000000000000000000000") } }
>
> db.u_info.insert({uid:1,name:"Falcon.C",address:"Beijing"});   
> db.u_info.insert({uid:2,name:"sexMan",address:"Wuhan"});   
> db.u_info.find();
{ "_id" : ObjectId("4b9cf280c84d7f20576c4df2"), "uid" : 1, "name" : "Falcon.C", "address" : "Beijing" }
{ "_id" : ObjectId("4b9cf284c84d7f20576c4df3"), "uid" : 2, "name" : "sexMan", "address" : "Wuhan" }插入了2条记录，我们来把uid设置为索引字段：

CODE:

> db.u_info.ensureIndex({uid:1});
> db.u_info.ensureIndex({name:1});
> db.system.indexes.find();
{ "name" : "_id_", "ns" : "user.u_info", "key" : { "_id" : ObjectId("000000000000000000000000") } }
{ "name" : "_id_", "ns" : "user.u_setting", "key" : { "_id" : ObjectId("000000000000000000000000") } }
{ "ns" : "user.u_info", "key" : { "uid" : 1 }, "name" : "uid_1" }
{ "ns" : "user.u_info", "key" : { "name" : 1 }, "name" : "name_1" }
>这时我们看到多了刚才我们设置的那个字段，这样在查询的时候，如果查询条件有uid字段或name字段，则走索引来进行查询

有索引：

CODE:

> db.u_info.find({name:"Falcon.C"});
{ "_id" : ObjectId("4b9cf280c84d7f20576c4df2"), "uid" : 1, "name" : "Falcon.C", "address" : "Beijing" }
> db.u_info.find({name:"Falcon.C"}).explain();
{
        "cursor" : "BtreeCursor name_1",
        "startKey" : {
                "name" : "Falcon.C"
        },
        "endKey" : {
                "name" : "Falcon.C"
        },
        "nscanned" : 1,
        "n" : 1,
        "millis" : 0,
        "allPlans" : [
                {
                        "cursor" : "BtreeCursor name_1",
                        "startKey" : {
                                "name" : "Falcon.C"
                        },
                        "endKey" : {
                                "name" : "Falcon.C"
                        }
                }
        ]
}删除索引后：

CODE:

> db.system.indexes.find();                  
{ "name" : "_id_", "ns" : "user.u_info", "key" : { "_id" : ObjectId("000000000000000000000000") } }
{ "name" : "_id_", "ns" : "user.u_setting", "key" : { "_id" : ObjectId("000000000000000000000000") } }
{ "ns" : "user.u_info", "key" : { "uid" : 1 }, "name" : "uid_1" }
{ "ns" : "user.u_info", "key" : { "name" : 1 }, "name" : "name_1" }
> db.u_info.dropIndex("name_1")
{ "nIndexesWas" : 3, "ok" : 1 }
> db.u_info.find({name:"Falcon.C"}).explain();
{
        "cursor" : "BasicCursor",
        "startKey" : {

        },
        "endKey" : {

        },
        "nscanned" : 2,
        "n" : 1,
        "millis" : 0,
        "allPlans" : [
                {
                        "cursor" : "BasicCursor",
                        "startKey" : {

                        },
                        "endKey" : {

                        }
                }
        ]
}
> db.system.indexes.find();                  
{ "name" : "_id_", "ns" : "user.u_info", "key" : { "_id" : ObjectId("000000000000000000000000") } }
{ "name" : "_id_", "ns" : "user.u_setting", "key" : { "_id" : ObjectId("000000000000000000000000") } }
{ "ns" : "user.u_info", "key" : { "uid" : 1 }, "name" : "uid_1" }通过以上可以看出，查询的条件中有索引时，查询走BtreeCursor 的索引，而没有索引时走BasicCursor


通常需要索引的字段是：
1.唯一键 _id 是默认被设置为索引的
2.需要被查找的字段应该建立索引,比如在find()里面的字段
3.需要被排序的字段应该建立索引。比如在sort()里面的字段

以上就是MongoDB的索引操作